1. Field of the Invention
The present invention relates to image processing for updating tone correction characteristics.
2. Description of the Related Art
A multi-functional peripheral equipment (MFP) which comprises copy, printer, and facsimile functions together suffers variations of the density characteristics of an output image due to factors of use environment, use frequency, and the like. For example, an electrophotography process includes processes, i.e., formation of a latent image on a photosensitive body by laser exposure, development of the latent image by toner, transfer of a toner image onto a print sheet, and fixing of toner by heat. These processes are susceptible to the influence of the temperature or humidity around the MFP and aging of its components, and a change in toner amount to be fixed on the print sheet causes density variations of an output image.
Variations of the density characteristics of an output image due to the environment (temperature and humidity), use frequency, and the like are not unique to the electrophotography system, but similarly occur in other systems such as an ink-jet printing system, thermal transfer system, and the like.
As a technique for reducing density variations, a method of generating a density correction table according to the density variations of an output image and correcting the density characteristics of image data is known.
FIG. 1 shows the density characteristics of an output unit of an image forming apparatus. The ordinate plots the output density: the value “0” represents white and the value “255” represents solid black. The abscissa plots data to input to the output unit: the value “0” represents white and the value “255” represents black. Ideal density characteristics of the output unit are linear characteristics indicated by a broken line 1400. When input data changes linearly, the density of a printout desirably changes linearly. However, the density characteristics of the output unit are influenced by the environment and use frequency, and exhibit a change indicated by a solid curve 1401, 1402, or 1403. Therefore, in order to make the density characteristics of a printout linear, the input data must be corrected using a tone correction table.
FIG. 2 is a graph for explaining a tone correction table. The ordinate plots output data: the value “0” represents white and the value “255” represents black. The abscissa is the same as that in FIG. 1.
When the output unit exhibits the density characteristics indicated by the solid curve 1401, correction characteristics 1501 symmetric to the axis of the broken line 1400 (or 1500) are set. Likewise, when the solid curve 1402 represents the density characteristics, correction characteristics 1502 are set. When the solid curve 1403 represents the density characteristics, correction characteristics 1503 are set. The tone correction table has input and output values of the correction characteristics 1501, 1502, and 1503 as table values. By correcting input data using the tone correction table, linear output density can be realized.
There are some methods of obtaining the correction characteristics. For example, an automatic tone correction method or the like is known. In this method, tone patches of Y, M, C, and K colors are printed as a sample chart, and the printed sample chart is read. Linear correction characteristics are created for Y, M, C, and K, respectively, and are set in a tone conversion (gamma conversion) unit.
A recent image forming apparatus has a plurality of halftone processing methods. For example, the density characteristics of an image binarized by error diffusion are different from those of an image binarized by halftone screen (dithering). Therefore, density correction using the tone correction table executes a method of correcting image data by creating a tone correction table corresponding to each halftone processing method.
Automatic tone correction of an image forming apparatus having a plurality of halftone processing methods requires operations for printing sample charts for respective halftone processing methods, and sequentially reading these sample charts. A controller that controls the automatic tone correction determines the halftone processing methods corresponding to the sample charts based on the read order of the sample charts, and reflects them to tone correction tables corresponding to the halftone processing methods. For example, upon execution of the automatic tone correction for three different halftone processing methods, a sample chart is printed and read for halftone processing methods. Therefore, a total of three print and read processes are required.
Especially, since the recent copying machine and MFP have high image quality and the halftone processing methods are diversifying, if the aforementioned automatic tone correction is carried out, a considerably long operation time is required. If automatic tone correction processes corresponding to many types of halftone processing methods are continuously executed, human errors such as wrong sample charts to be read, wrong directions of sample charts to be read, and the like readily occur.